1. Field of the Invention
The present invention relates to a volumetric ultrafiltration system and its method of use, and more particularly, concerns an apparatus and method for controlling ultrafiltration during hemodialysis.
2. Description of the Prior Art
Ultrafiltration is the procedure during hemodialysis wherein excess water is removed from the blood. It is well-known to achieve satisfactory ultrafiltration by maintaining the dialysate pressure within the dialyzer lower than that of the blood pressure. During this procedure, while excess water in the blood is removable, the rate of ultrafiltration is a critical factor, since rapid removal of water from the blood may traumatically affect the patient. Various solutions to the control of the rate of ultrafiltration have been proposed, one of which is found in U.S. Pat. No. 4,021,341.
More recently, ultrafiltration control has been achieved by utilization of a volumetric system relying upon a principle of volume conservation. One such system is described in U.S. Pat. No. 4,209,391. In the patented apparatus, a known and equal quantity of fluid is moved into and out of the dialyzer by two matched positive displacement pumps. As dialysate is moved to the dialyzer, a third pump extracts the programmed amount of dialysate from the fresh dialysate supply. The spent dialysate line demands a fixed quantity of fluid and the difference that is drawn off the fresh dialysate supply is made up by ultrafiltrate drawn across the dialyzer membrane. In U.S. Pat. No. 4,209,391, the patentees recognized that there are certain errors which arise in attempts to monitor dialysate volumes. These patentees contended that the major source of such errors is the inclusion of gases in the circulating dialysate. Based on that premise, the solution to that problem as proposed in U.S. Pat. No. 4,209,391 was to remove all gases which may enter the closed circuit in the subatmospheric pressure portion from all of the spent dialysate in order to achieve actual liquid integrity in a closed circuit.
However, in an article entitled "The Governing Equation Describing The Transient Characteristics Of Hemodialysis Ultrafiltration," by Massie, H. L., and Chen, P. I., Thirtieth ACEMB, Los Angeles, Calif., 5-9 Nov. 1977, page 158, the authors analyzed the process of hemodialysis specifically with respect to machine disturbances and patient blood pressure and movement. The authors concluded that dialyzer compliance is the primary source of error in the measurement of ultrafiltration, and that air displacement of fluid volume is a secondary factor. While the above-identified authors suggested an equation useful for dialysis simulation, no tangible or physical adaptation of their theory was proposed at that time. Specifically, dialyzer compliance is a reference to dialyzer membrance compliance and is a function of mean transmembrane pressure (TMP) as well as the time function change in TMP. An overall characterization of the term "compliance" is the elasticity of the closed system. Accordingly, inasmuch as the major source of error in dialysate volumetric monitoring has been shown to be the compliance factor, the hemodialysis ultrafiltration control system should be designed to account for compliance in order to accurately monitor and/or control ultrafiltration. It is to such a system that the present invention is directed in order to solve the major errors in monitoring dialysate volumes.